Robots are used in many industrial applications to place an article at a predetermined location in a workspace as part of a manufacturing process. Within the electronics industry, robots are being employed with increasing frequency to place components on a printed circuit board, repeatedly and with high accuracy. Currently, there is a trend within the electronics industry toward increasing the functionality of electronic components while reducing their overall size. Reduction in the overall component size has led to a reduction in the size and pitch of the component's leads and a concurrent reduction in the size and pitch of the metallized areas on the circuit board to which such leads are bonded. As the pitch and size of the component leads and the metallized circuit board areas have become smaller, the amount of error permitted when placing the components on the boards has likewise decreased. For some very fine pitch components, the maximum allowable placement error is often smaller than the lowest obtainable placement error of the robot which is to carry out such placement.
The accuracy by which a robot can place an electronic component or other type of article at a predetermined location within a workspace (such as on a circuit board) is generally measured by the positional error incurred by when the robot's manipulator is displaced from one location to another location in the workspace. The accuracy of a robot should not be confused with its repeatability, that is, the ability of the robot to return its manipulator to the same position every time. Most robots have very high repeatability, but it is the robot's accuracy which may vary because of many factors, including design, as well as environmental factors, such as thermal expansion.
In many instances, a machine vision system, comprised of a camera, and a processor for processing the camera's image, is employed to improve the placement accuracy of a robot. For example, a machine vision system can be employed to establish the distance (offset) between the robot's manipulator and a fiducial situated at a known location on a circuit board, after the manipulator has been positioned in registration with the fiducial. The measured offset represents the robot's positional error and can be used to compensate the robot to improve its placement accuracy. The disadvantage of this approach is that, in practice, the robot's position error is not a constant and, in fact, varies with the position of the robot's manipulator within the workspace.
The placement accuracy of the robot can also be improved with the aid of a machine vision system when employed to measure the offset between the location where the component is to be placed, and the actual manipulator position once the manipulator is placed in registration with the placement location. The measured offset error can be fed back to the robot to cause it to re-position its manipulator to reduce the offset. Depending on the degree of positional accuracy required, the process of measuring the offset and displacing the robot's manipulator accordingly to reduce the error may be repeated several times until the error is less than a predetermined tolerance factor. Improving the robot's accuracy in this fashion is cumbersome since each time a component is placed, the offset must be precisely measured, and then fed back the robot in order to reposition its manipulator. For each article placed, a small amount of time is spent performing these operations. When placing a large number of components, the overall amount of time spent measuring the robot's positional offset and moving the manipulator accordingly can become significant.
Thus, there is need for a technique for increasing the placement accuracy of a robot which does not incur the aforementioned disadvantages.